1. Field of the Invention
The present invention relates to a surface emitting laser, a surface emitting laser array, and an image forming apparatus including a surface emitting laser.
2. Description of the Related Art
A VCSEL can emit a beam in a direction perpendicular to its semiconductor substrate and therefore can easily be applied to a two-dimensional array. Parallel processing of multiple beams emitted by a two-dimensional VCSEL array allows for a higher density and a higher speed. Thus, a VCSEL is expected to be used for various industrial applications.
In a VCSEL, to efficiently supply an electric current to an active layer, AlxGa1-xAs (hereinafter also referred to as “AlGaAs”) having a high Al content is selectively oxidized to form a current confinement structure. A typical diameter of current confinement is generally about 3 μm for single transverse mode operation.
However, such a small confinement diameter results in a small active area and therefore greatly reduces the output of a laser device.
To perform single transverse mode oscillation even at a larger confinement diameter, IEEE Photonics Technology Letters, vol. 12, No. 8, 2000, p. 939, proposes to increase the diffraction loss of a higher-order transverse mode by increasing the cavity length. The structure of a surface emitting laser device described in this IEEE document will be described below with reference to FIG. 2.
A lower semiconductor multilayer reflector 220 is disposed on a GaAs substrate 210. The lower semiconductor multilayer reflector 220 includes alternately stacked low and high refractive index sublayers. Each of the low refractive index sublayers and the high refractive index sublayers has an optical thickness of λ/4. The optical thickness of a layer is the product of the thickness of the layer and the refractive index of the material forming the layer. The wavelength λ refers to the oscillation wavelength. A multilayer reflector is also referred to as a distributed Bragg reflector (DBR).
A GaAs spacer layer 230 having a thickness larger than usual is disposed on the lower semiconductor multilayer reflector 220. A lower cladding layer 240, an active layer 250 including quantum wells, and an upper cladding layer 260 are disposed in this order on the spacer layer 230. An upper semiconductor multilayer reflector 270 is disposed on the upper cladding layer 260. The upper semiconductor multilayer reflector 270 includes alternately stacked low and high refractive index sublayers.
The spacer layer 230 is formed only of GaAs and has a length in the range of 2 to 8 μm. Generally, in surface emitting lasers, the optical thickness of a cavity defined by upper and lower DBRs is designed to be about one or two wavelengths. For example, in a 980 nm laser described in the above-mentioned IEEE document, the cavity length is about 0.3 μm for a one-wavelength cavity and about 0.6 μm for a two-wavelength cavity.
In a surface emitting laser that includes a cavity having an optical thickness of one or two wavelengths, the laser oscillates in a higher-order mode, as well as a fundamental mode, at a diameter of oxidation confinement larger than 3 or 4 μm.
Because the surface emitting laser described in the above-mentioned IEEE document includes a spacer having a length as large as about 8 lam, oscillation in a single fundamental transverse mode is achieved even at a diameter of oxidation confinement of 7 μm. In a surface emitting laser having a long cavity structure, the long distance between DBRs serving as mirrors causes a propagating beam to spread. As for a beam within a surface emitting laser device, the divergence angle for a beam of a higher-order mode is larger than that for a fundamental mode. Thus, in a surface emitting laser having a long cavity structure, a beam of a higher-order mode tends to have a large diffraction loss while propagating between the DBRs. Single transverse mode oscillation in a fundamental mode is therefore more easily performed in the surface emitting laser having a long cavity structure than in lasers not having a long cavity structure.
In a surface emitting laser device, heat generation in the device has a large influence on its optical output power. Hence, improvement in heat dissipation capacity is another technical issue. In particular, the temperature characteristics of an active layer formed of AlGaInP/GaInP, in a surface emitting laser for a red band emission ranging from 630 to 690 nm are inferior to the temperature characteristics of an active layer in an infrared semiconductor laser. Thus, the heat dissipation capacity is more important in the surface emitting laser with an AlGaInP/GaInP active layer.
U.S. Patent Application Publication No. 2005/0271113 discloses a structure in which a heat conductive layer having an optical thickness of an integral multiple of λ/2 is disposed under a cladding layer. The heat conductive layer improves the heat dissipation capacity, thus increasing the laser output. The heat conductive layer is formed of GaAs, AlAs, or InP.
As described above, the practical utilization of a surface emitting laser for a red band emission requires single transverse mode oscillation and improved heat dissipation capacity. The present inventors used a thick AlAs film, which exhibits low band-to-band absorption, as a spacer layer to achieve single transverse mode oscillation and improved heat dissipation capacity.
However, as a result of extensive research, the present inventors found that an AlAs film having a thickness necessary for transverse mode control, for example, 1 μm or more, was difficult to grow. More specifically, the crystal growth of an AlAs monolayer film having a thickness of at least 1 μm resulted in such a rough crystal surface that the AlAs monolayer film was not acceptable as a substrate for use in a laser, especially for the growth of active layer.
In conclusion, while a laser device that includes an AlAs layer having a thickness below 1 μm may be produced to improve the heat dissipation capacity, an AlAs layer having a thickness necessary for single transverse mode oscillation of a long cavity, that is, at least 1 μm, is difficult to produce.